Laminated veneer lumber ("LVL") is a structural wood composite incorporating characteristics of both plywood and sawn lumber. Like plywood, LVL is made of adhesively bonded wood veneer sheets. But, whereas plywood is cross-banded, the veneer in LVL is bonded with the grain in each sheet running parallel, similar to sawn lumber. Because any veneer defects tend to be distributed evenly in LVL, dimensional flaws characteristic of sawn lumber are minimized, resulting in improved LVL structural properties.
In LVL manufacture, the ends of the individual veneer sheets are joined by either a lap, butt or scarf joint, with the joints being staggered throughout the cross-section of the LVL "billet". Glue is applied to the opposed, outer faces of each veneer sheet, excepting the two outermost surfaces. The glued sheets are laid atop one another to form the billet, which is then compressed and heated, firmly bonding the veneer into a single piece of LVL having superior strength characteristics.
LVL may be made in a fixed length press or in a continuous layup press. In a fixed length press the entire billet fits between a pair of opposed platens which compress and heat the billet. In a continuous layup press only part of the billet fits between the platens; the remaining part of the billet protrudes outside the platens.
A continuous layup press cures the billet in stages. After a controlled interval, the platens are opened and the billet is advanced longitudinally with respect to the platens. This removes the cured part of the billet from the press and places the adjacent uncured part between the platens for another controlled interval. The process is repeated until a desired length of the billet is cured. That length can then be cut of the billet for use. Alternatively, the entire billet can be cured in stages as aforesaid to yield a single piece of LVL.
Continuous layup presses are more versatile than fixed length presses: a comparatively short continuous layup press can make LVL in varying lengths, any of which may be considerably longer than the press platens. But, the maximum length of LVL made by a fixed length press is restricted by the length of the press platens.
LVL presses typically use hot oil heating systems. Alternatively, dielectric heating systems which make use of an effect in which the opposed press platens act as the plates of a very large parallel plate capacitor are sometimes used. The platens are coupled to a radio frequency ("RF") energy source, whereby RF energy is radiated between the platens and thus through the billet. This heats the wood and promotes curing of the glue as the platens are compressed toward one another.
Although it would be desirable to employ dielectric heating in a continuous layup press, problems have been encountered in doing so. RF energy "leaks" from between the RF heating platens and flows laterally along the glue lines within the uncured billet portion which protrudes outside the RF heating platens. Such leakage causes premature curing and/or burning of the glue in the billet portion outside the heating platens, preventing proper compression of that portion when it is subsequently advanced between the platens for heating and compression.
The present invention overcomes the foregoing problems, thus making practical the use of a dielectric heating effect in a continuous layup LVL press.